PLANES OF MOVEMENT
PRINCIPLES
OF LOCATION:
In
interchangeable manufacture, when a work piece is placed on a machine table,
spindle, jig or fixture it must be properly positioned before it is clamped. It
is the duty of the location system to provide the necessary constrain so that
the work piece is correctly located and all possible movements of the work
piece are restrained. The term 'location' refers, to the dimensional and
positional relationship between "the work piece and the cutting tool used
in the machine. After the correct location, clamping is essential. The same
mechanism usually incorporates both the locating and clamping mechanism. Common
surfaces to be located have the forms of planes, cylinders, cones or some
special type of surface, for which locating devices have to be devised. The
function of damping is to exert a force to work piece against the locating
surfaces and hold it there against the action of cutting force.
TWELVE
DEGREES OF FREEDOM:
A
work piece in space which is free to move in any direction can have twelve such
directions. It can move in either of two opposed directions along three
mutually perpendicular axes and may rotate in either of two opposed directions
around each axes.ie, clockwise and counter clockwise.
Each
direction of movement is considered one degree of freedom. Thus there are
twelve degrees of freedom for any work piece in space. To locate a work piece
accurately it must- be confined to restrict it against movements in any of the
twelve degrees of freedom except" those called for by the operation. Work
piece can be accurately and positively confined in a jig or fixture if the
above condition is satisfied. The 12 degrees of freedom of a piece is shown in
figure 2.1.
Fig.
- 2.1. 12 Degrees of Freedom of an Object
CONSIDER THE POSSIBLE
MOVEMENTS OF THE FREE BODY SHOWN WITH RESPECT TO THESIX MUTUALLY PERPENDICULAR
AXES" X-X’, Y-Y' & Z-Z' "
IT CAN:
1. MOVE ALONG X-X' -IN TWO
DIRECTIONS - 1, 2
2. MOVE ALONG Y-Y' -IN TWO DIRECTIONS -
3, 4
3. MOVE ALONG Z-Z'- IN TWO DIRECTIONS -
5, 6
SIX
FREEDOMS OF TRANSLATION
4. ROTATE ABOUT X-X'-IN CLOCKWISE AND
ANTI-CLOCKWISE- 7, 8
5. ROTATE ABOUT Y-Y'-IN CLOCKWISE AND
ANTI-CLOCKWISE- 9, 10
6. ROTATE ABOUT Z-Z'-IN CLOCKWISE AND
ANTI-CLOCKWISE- 11, 12
SIX FREEDOMS OF ROTATION
TOTAL: - TWELVE DEGREES OF FREEDOM
THE DUTY
OF THE LOCATION SYSTEM
The
location system must, in conjunction with the clamping system completely
constrain the work piece or eliminate as many of the 12 degrees of freedom as
is necessary for the operation to be completed with the required accuracy.
THE
CHOICE OF LOCATION SYSTEM
The
requirements of the location system depend upon the operation being performed,
and upon the work piece before the operation.
Fig-2.2 illustrates
three stages in the machining of a part. When this part is positioned for stage
2 machining it does not need to be controlled about the XX axis because
it is symmetrical about that axis, but it must be completely constrained end
positioned for stage 3 machining because it is no longer symmetrical about the
axis after hole A is machined at stage 2.
While
choosing location points, the most effective location point must be selected.
The cylinder is the best location shape because a cylindrical locator is the
last difficult to produce, and because a single locator of this shape will
eliminate 10 the 12 degrees of freedom. Also the ease of loading and unloading
of the work piece must also be considered. This is illustrated in fig-2.3,
which shows two methods of machining a work piece. In operation 2 there is a
choice between machining hole 'L' and hole 'H'. As the work piece must be
constrained when it is sectioned for operation 3, two locators are necessary.
If method 'A' is used, the locators for operation 3 will be parallel and easily
seen during loading, but if method is used, the locator that engages hole 'H'
will not be seen easily, and must be intractable so that the work piece
can be loaded. Method 'A' is obviously the better method.
LOCATION
TWO METHODS OF MACHINING OF A WORKPIECE
(LOCATIONS SHOWN BY HEAVY LINES)
COMMENTS